Conventionally, in a tandem type color copier, it has been a common practice that each photosensitive drum for respective color components, namely black, magenta, cyan, yellow, is combined together with the respective toner tank to construct a photosensitive unit, and each photosensitive unit is arranged along a feed direction of the copy sheet. Then a toner image is developed onto the respective photosensitive units to perform a multiple toner transferring operation in series, i.e., a second toner image onto a first toner image formed on the photosensitive unit, until a forth toner image is transferred onto the previous toner image without color mis-matching to finally obtain a desired color image.
In the above described photo-sensitive drum unit, a rotary drive of the photo-sensitive drum is given from a power source of a main body of a copy machine via a drive transmission mechanism.
In addition, when the toner cartridge needs to be replaced or when the photosensitive drum needs to be replaced with a new one due to elapse of its available life, the photosensitive unit is so constructed that it can be detached from the drive transmission mechanism.
As the aforementioned drive transmission mechanism for transmitting the drive from the power source to the photosensitive drum unit, there are several types. One is a so-called direct drive transmission type in which a gear portion formed at the drum flange of the photo sensitive drum is meshed in a gear train driven from the power source and another type is a co-called drum shaft type in which four shafts are extended from the power source and the respective photosensitive drums are engaged with the respective drum shafts and the other type is a so-called coupling type in which the driven shaft of the photosensitive drum is connected to the drive shaft via a coupling.
In practical sense, the photosensitive unit as a replaceable part is designed to have manufacturing tolerances, thus at the time of replacement, such manufacturing tolerance has to be taken into consideration when connecting the driven shaft of the photosensitive drum to the drive shaft. In view thereof, it would be much easier when using a coupling for such connection between the drive shaft and the driven shaft of the photosensitive drum unit for the coupling by itself can absorb such manufacturing tolerances. Examples of this type of connection, namely a coupling connection, are such as a cruciform universal joint as shown in FIG. 6 and an Oldham's coupling as shown in FIG. 7.
Referring now to FIG. 6, two pins 2,3 orthogonary arranged from one another, are provided at a connecting plate 1 and a C-shaped arm 4,5 is pivotally fitted to the opposite ends of each pin 3,2 respectively. Thus when two shafts 6,7 to be coupled are intersecting each other at a certain angle, i.e., two shafts are not aligned on a straight line but are bent with respect to each other by a certain bending angle, such bending angle is absorbed by the coupling, thereby causing no adverse effect on the coupling status of those two shafts 6,7. In addition, the C-shaped arm 4 is slightly displaceable along the pin 2 and similarly the C-shaped arm 5 is slightly displaceable along the pin 3, thus such misalignment between the axes, S1 & S2, of the shaft 6 and the shaft 7, if occurred, can also be absorbed by this structure of the cruciform joint.
On the other hand, in the Oldham's coupling structure, a shaft 8 and a shaft 9 are respectively fixed onto a disk 10 and a disk 11 and a connecting disk 12 is interposed between the disks 10, 11. Furthermore, orthogonally crossing grooves are respectively formed on both surfaces of the disk 12 and a projection 13 formed on the disk 10 and a projection formed on the disk 11 are respectively engaged with grooves on the respective sides of the disk 12 in such a manner that the disk 10 is slidable along the extension direction of the projection 13 and the disk 11 is slidable along the extension direction of the projection 14. Thus, when the axis S3 of the shaft 8 and the axis S4 of the shaft 9 are not aligned from each other in some degree, this mis-alignment of the shaft axes is taken care by this coupling.
However, the cruciform joint of this conventional type is consisted of a great number of parts such as a pair of C-shaped arms 4,5, a connection plate 1, pins 2,3 and so forth, and thus has a complicated structure. Accordingly there has been a problem in assembling the drive and driven portions with this joint. On the other hand, with the Oldham's joint, the problem associated with the cruciform joint, difficulty in assembling, is solved, as the Oldham's joint is greatly simple in structure as compared with the cruciform joint. However, the Oldham's joint cannot cope with the shafts to be coupled, whose axes are not in parallel from each other.
In view of the above, there is certainly room for improvement in designing a coupling structure for use in coupling the power source of the main body of the copy machine and the photosensitive drum of the photosensitive unit.